The production of petroleum oil from underground deposits such as porous sandstone reservoirs is effected by drilling a well into such deposit. Since the pressure at the deposit is greater than the pressure at the surface of the well, the petroleum oil flows upwardly. The pumping of a well until the spontaneous flow is exhausted is termed "primary" production in the art and is generally characterized by recovering, on average, about 17% of the crude petroleum oil present in the desposit.
A substantial additional amount of oil can be recovered from the "exhausted" reservoir by flooding it with water by means of an injection well so as to displace the oil toward an adjacent producing well. This operation is termed "secondary" production and is widely practiced in order to recover an additional 15% of the total oil in the reservoir. The use of relatively sophisticated means to further enhance oil recovery from about 32% to about 35-38% is termed "tertiary" production. Thus, for every barrel recovered by primary and secondary production means, there are two barrels of oil left in the reservoir which invite recovery by tertiary production methods. A great deal of laboratory work and field work have been done over the past fifty years in order to develop and improve tertiary or enhanced oil recovery technology.
The recent dramatic increase in the cost of imported crude petroleum oil has generated even more interest in enhanced oil recovery. Among the methods that have been employed to secure tertiary production are surfactant flooding, polymer flooding, methane flooding, carbon dioxide flooding, and "fire" flooding. The last named method is the most widely employed and accounts for about 60% of enhanced oil recovery procedures. It involves subjecting the reservoir to higher temperatures in order to reduce viscosity while also providing a displacement medium to drive the released oil to the producing well. In one method of fire flooding an in situ combustion of some of the retained oil is achieved by pumping air or oxygen into the deposit through the injection well. It is also known to inject water for conversion to steam by partially quenching the underground fire. Alternatively, high pressure steam can be directly injected to drive the mobilized oil to a production well or the steam treatment is applied cyclically into a given well by alternating injection and production phases in the so called "huff and puff" mode of stream flooding.
In U.S. Pat. No. 2,412,765 propane and butane vapors are injected into a well and allowed to condense in the reservoir, thereby transferring a large quantity of energy into the reservoir. The mixture of condensed hydrocarbon vapors and condensed oil is then allowed to drain by gravity to the well bore of the producing well from which it is recovered by pumping. There is no positive displacement of the mixture as would be the case if a liquid solvent were injected under sufficient pressure to maintain it in the liquid state.